GAP COLONIZATION AS A SOURCE OF GRASSLAND COMMUNITY CHANGE - EFFECTS OF GAP SIZE AND GRAZING ON THE RATE AND MODE OF COLONIZATION BY DIFFERENT SPECIES

Grazers may bring about vegetation change in pastures through effects on the creation and colonization of gaps. The natural colonization of three sizes of artificial gap in a species-poor, fertile pasture was m onitored in an experiment in which two seasons of sheep grazing were a pplied, each at two levels: spring (grazed or ungrazed) and summer (ha rd or light grazed). The experiment had a 2 x 2 factorial design with two blocks in eight paddocks. Gap filling was slow; after 50 weeks the ramet density of the smallest gaps was 71% of that of the surrounding sward. Seedling establishment was the dominant mode of colonization, accounting for 59% of colonizing ramets. The remaining colonists were from clonal ingrowth. Smaller gaps filled fastest, having higher clona l ramet densities than the larger gaps. Gap size had complex effects o n seedling colonization. Over the whole gap area there was a non-signi ficant trend for increased densities of seed-derived plants in smaller gaps, but in just the central area of the gap, increased gap size inc reased the density and size of seed-derived plants. There were no spri ng grazing or grazing treatment x gap size interactions. Harder summer grazing did not affect gap filling rates but it decreased seedling de nsities and increased clonal ramet densities, possibly by reducing flo wering. Species differed in gap colonization ability, measured by the change in a species' frequency between the surrounding sward and the c olonized gap. Species also differed significantly in the ratio of seed -derived to clonal colonizing ramets, ranging from almost complete clo nal colonizers to almost complete seedling colonizers. Species with a higher proportion of colonizing ramets derived from seed had higher co lonization abilities than more clonal species. Increased gap size incr eased the frequencies of some species and decreased those of others. B oth spring and summer grazing treatments affected the frequencies in t he gaps of some species. There were no interaction effects of gap size and grazing treatment on gap species composition. Treatment effects o n species with high proportions of seed-derived ramets were due only t o effects on the frequencies of seed derived ramets and treatment effe cts on more clonal species were due to effects on the frequencies of c lonal ramets. These results show that grazing may bring about vegetati on change by its effects on the rate of gap creation because colonized gaps have different species frequencies to the closed sward. However, these gap effects are complex; the frequencies of some common species were decreased in gaps but those of other common species were increas ed, and most rarer species were decreased in gaps. Gap size and grazin g treatment effects show that the effects of gap creation on vegetatio n change will be dependent on the sizes of gaps created.